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RFC 6538 - The Host Identity Protocol (HIP) Experiment Report


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Internet Research Task Force (IRTF)                         T. Henderson
Request for Comments: 6538                            The Boeing Company
Category: Informational                                        A. Gurtov
ISSN: 2070-1721                                       University of Oulu
                                                              March 2012

           The Host Identity Protocol (HIP) Experiment Report

Abstract

   This document is a report from the IRTF Host Identity Protocol (HIP)
   research group documenting the collective experiences and lessons
   learned from studies, related experimentation, and designs completed
   by the research group.  The document summarizes implications of
   adding HIP to host protocol stacks, Internet infrastructure, and
   applications.  The perspective of a network operator, as well as a
   list of HIP experiments, are presented as well.  Portions of this
   report may be relevant also to other network overlay-based
   architectures or to attempts to deploy alternative networking
   architectures.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the consensus of the IRTF HIP
   Research Group of the Internet Research Task Force (IRTF).  Documents
   approved for publication by the IRSG are not a candidate for any
   level of Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6538.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
      1.1. What is HIP? ...............................................3
      1.2. Terminology ................................................4
      1.3. Scope ......................................................4
      1.4. Organization ...............................................5
   2. Host Stack Implications .........................................6
      2.1. Modifications to TCP/IP Stack Implementations ..............6
           2.1.1. ESP Implementation Extensions .......................8
      2.2. User-Space Implementations .................................9
      2.3. Issues Common to Both Implementation Approaches ............9
           2.3.1. User-Space Handling of HITs .........................9
           2.3.2. Opportunistic Mode .................................10
           2.3.3. Resolving HITs to Addresses ........................12
           2.3.4. IPsec Management API Extensions ....................12
           2.3.5. Transport Protocol Issues ..........................12
           2.3.6. Legacy NAT Traversal ...............................14
           2.3.7. Local Management of Host Identity Namespace ........14
           2.3.8. Interactions with Host Firewalls ...................15
      2.4. IPv4 versus IPv6 Issues ...................................15
      2.5. What Have Early Adopters Learned from Experience? .........16
   3. Infrastructure Implications ....................................17
      3.1. Impact on DNS .............................................17
      3.2. HIP-Aware Middleboxes .....................................17
      3.3. HIT Resolution Infrastructure .............................18
      3.4. Rendezvous Servers ........................................18
      3.5. Hybrid DNS-DHT Resolution .................................19
   4. Application Implications .......................................20
      4.1. Non-Intrusive HIP Insertion ...............................20
      4.2. Referrals .................................................20
      4.3. Latency ...................................................21
   5. Network Operator's Perspective .................................21
      5.1. Management of the Host Identity Namespace .................21
      5.2. Use of ESP Encryption .....................................22
      5.3. Access Control Lists Based on HITs ........................22
      5.4. Firewall Issues ...........................................23
   6. User Privacy Issues ............................................24
   7. Experimental Basis of This Report ..............................25
   8. Related Work on ID-Locator Split ...............................27
   9. Security Considerations ........................................28
   10. Acknowledgments ...............................................28
   11. Informative References ........................................29

1.  Introduction

   This document summarizes the work and experiences of the IRTF's Host
   Identity Protocol research group (HIP-RG) in the 2004-2009 time
   frame.  The HIP-RG was chartered to explore the possible benefits and
   consequences of deploying the Host Identity Protocol architecture
   [RFC4423] in the Internet and to explore extensions to HIP.

   This document was developed over several years as the main charter
   item for the HIP research group, and it has received inputs and
   reviews from most of the active research group participants.  There
   is research group consensus to publish it.

1.1.  What is HIP?

   The Host Identity Protocol architecture introduces a new namespace,
   the "host identity" namespace, to the Internet architecture.  The
   express purpose of this new namespace is to allow for the decoupling
   of identifiers (host identities) and locators (IP addresses) at the
   internetworking layer of the architecture.  The contributors to HIP
   have expected that HIP will enable alternative solutions for several
   of the Internet's challenging technical problems, including
   potentially host mobility, host multihoming, site multihoming, IPv6
   transition, NAT traversal, and network-level security.  Although
   there have been many architectural proposals to decouple identifiers
   and locators over the past 20 years, HIP is one of the most actively
   developed proposals in this area [book.gurtov].

   The Host Identity Protocol itself provides a rapid exchange of host
   identities (public keys) between hosts and uses a Diffie-Hellman key
   exchange that is compliant with Sigma ("SIGn-and-MAc") to establish
   shared secrets between such endpoints [RFC5201].  The protocol is
   designed to be resistant to Denial-of-Service (DoS) and Man-in-the-
   Middle (MitM) attacks, and when used together with another suitable
   security protocol, such as Encapsulated Security Payload (ESP)
   [RFC4303], it provides encryption and/or authentication protection
   for upper-layer protocols such as TCP and UDP, while enabling
   continuity of communications across network-layer address changes.

   A number of Experimental RFC specifications were published by the
   IETF's HIP working group, including the HIP base protocol [RFC5201],
   ESP encapsulation [RFC5202], registration extensions [RFC5203], HIP
   rendezvous servers [RFC5204], DNS resource records [RFC5205], and
   mobility management [RFC5206].  Host identities are represented as
   Overlay Routable Cryptographic Hash Identifiers (ORCHIDs) [RFC4843]
   in Internet protocols.  Additionally, the research group published
   one RFC on requirements for traversing NATs and firewalls [RFC5207]

   and the working group later published specification text for legacy
   NAT traversal [RFC5770].  As of this writing, work has commenced on
   moving the above specifications to Standards Track status.

1.2.  Terminology

   The terms used in this document are defined elsewhere in various
   documents.  In particular, readers are suggested to review Section 3
   of [RFC4423] for a listing of HIP-specific terminology.

1.3.  Scope

   The research group has been tasked with producing an "experiment
   report" documenting the collective experiences and lessons learned
   from other studies, related experimentation, and designs completed by
   the research group.  The question of whether the basic identifier-
   locator split assumption is valid falls beyond the scope of this
   research group.  When indicated by its studies, the HIP-RG can
   suggest extensions and modifications to the protocol and
   architecture.  It has also been in scope for the RG to study, in a
   wider sense, what the consequences and effects that wide-scale
   adoption of any type of separation of the identifier and locator
   roles of IP addresses is likely to have.

   During the period of time when the bulk of this report was drafted
   (2004-2009), several research projects and open source software
   projects were formed to study HIP.  These projects have been
   developing software enabling HIP to be interoperable according to the
   Experimental RFCs as well as supporting extensions not yet specified
   by RFCs.

   The research group has been most active in two areas.  First and
   foremost, the research group has studied extensions to HIP that went
   beyond the scope and charter of the IETF HIP working group and the
   set of RFCs (RFC 5201-5206) initially published in April 2008.  Some
   of this work (NAT traversal, certificate formats for HIP, legacy
   application support, and a native sockets API for HIP) ultimately
   flowed into the IETF HIP working group upon its recharter in 2008.
   Other extensions (e.g., HIP in the Internet Indirection
   Infrastructure (i3) overlay, use of distributed hash tables for HIT-
   based (Host Identity Tag) lookups, mobile router extensions, etc.)
   are either still being worked on in the research group or have been
   abandoned.  Most of the energy of the research group during this time
   period has been in studying extensions of HIPs or the application of
   HIP to new problem domains (such as the Internet of Things).  Second,
   the research group has discussed the progress and outcome of the
   implementations and experiments conducted so far, as well as
   discussing perspectives from different participants (end users,

   operators, enterprises) on HIP deployment.  It is this latter
   category of work (and not the extensions to HIP) on which this report
   is focused.

   Finally, the research group was chartered to study, but did not
   rigorously study (due to lack of inputs), the following issues:

   o  Objective comparisons of HIP with other mechanisms (although the
      research group did hold some discussions concerning the relation
      of HIP to other efforts such as the End-Middle-End (EME) research
      group, the Routing research group (RRG), and shim6-based
      protocols).

   o  Large scale deployments (thousands of hosts or greater).

   o  Exploration of whether introducing an identity-locator mechanism
      would be architecturally sound, deployed at wide scale.

   o  Changes to the HIP baseline architecture and protocol or other
      identity-locator separation architectures.

1.4.  Organization

   In this report, we summarize the collective experience of early
   implementers and adopters of HIP, organized as follows:

   o  Section 2 describes the implications of supporting HIP on an end
      host.

   o  Section 3 covers a number of issues regarding the deployment of
      and interaction with network infrastructure, including middlebox
      traversal, name resolution, Access Control Lists (ACLs), and HIP
      infrastructure such as rendezvous servers.

   Whereas the two previous sections focus on the implementation and
   deployment of the network plumbing to make HIP work, the next three
   focus on the impact on users and operators of the network.

   o  Section 4 examines how the support of HIP in the host and network
      infrastructure affects applications; whether and how HIP provides
      benefits or drawbacks to HIP-enabled and legacy applications.

   o  Section 5 provides an operator's perspective on HIP deployment.

   o  Section 6 discusses user privacy issues.

   In closing, in Section 7, we list the experimental activities and
   research that have contributed to this report, and in Section 8 we
   briefly summarize related work.

2.  Host Stack Implications

   HIP is primarily an extension to the TCP/IP stack of Internet hosts,
   and, in this section, we summarize some experiences that several
   implementation groups have encountered in developing this extension.
   Our discussion here draws on experience of implementers in adding HIP
   to general-purpose computing platforms such as laptops, desktops,
   servers, and PDAs.  There are two primary ways to support HIP on such
   an end host.  The first is to make changes to the kernel
   implementation to directly support the decoupling of identifier and
   locator.  Although this type of modification has data throughput
   performance benefits, it is also the more challenging to deploy.  The
   second approach is to implement all HIP processing in the user-space
   and configure the kernel to route packets through user-space for HIP
   processing.

   The following public HIP implementations are known and actively
   maintained:

   o  HIP4BSD (http://www.hip4inter.net) -- FreeBSD kernel modifications
      and user-space keying daemon;

   o  HIPL (http://hipl.hiit.fi) -- Linux kernel and user-space
      implementation;

   o  OpenHIP (http://www.openhip.org) -- User-space keying daemon and
      packet processing for Linux, Windows XP/Vista/7, and Apple OS X.

   In the following, we first describe issues specific to the way in
   which HIP is added to a stack, then we discuss general issues
   surrounding both implementation approaches.

2.1.  Modifications to TCP/IP Stack Implementations

   In this section, we focus on the support of HIP assuming the
   following:

   o  HIP is implemented by directly changing the TCP/IP stack
      implementation.

   o  Applications (using the sockets API) are unaware of HIP.

   A HIP implementation typically consists of a key management process
   that coordinates with an IPsec-extended stack, as shown in Figure 1.
   In practice, HIP has been implemented entirely in the user-space,
   entirely in the kernel, or as a hybrid with a user-space key
   management process and a kernel-level ESP.

    +--------------------+                       +--------------------+
    |                    |                       |                    |
    |                    |                       |                    |
    |   +------------+   |                       |   +------------+   |
    |   |    Key     |   |         HIP           |   |    Key     |   |
    |   | Management | <-+-----------------------+-> | Management |   |
    |   |  Process   |   |                       |   |  Process   |   |
    |   +------------+   |                       |   +------------+   |
    |         ^          |                       |         ^          |
    |         |          |                       |         |          |
    |         v          |                       |         v          |
    |   +------------+   |                       |   +------------+   |
    |   |   IPsec-   |   |        ESP            |   |   IPsec-   |   |
    |   |  Extended  |   |                       |   |  Extended  |   |
    |   |   Stack    | <-+-----------------------+-> |   Stack    |   |
    |   |            |   |                       |   |            |   |
    |   +------------+   |                       |   +------------+   |
    |                    |                       |                    |
    |                    |                       |                    |
    |     Initiator      |                       |     Responder      |
    +--------------------+                       +--------------------+

                      Figure 1: HIP Deployment Model

   Figure 2 summarizes the main implementation impact of supporting HIP,
   and is discussed further in subsequent sections.  To enable HIP
   natively in an implementation requires extensions to the key
   management interface (here depicted as PF_KEY API [RFC2367]) with the
   security association database (SAD) and security policy database
   (SPD).  It also requires changes to the ESP implementation itself to
   support BEET-mode (Bound End-to-End Tunnel) processing [BEET-MODE],
   extensions to the name resolution library, and (in the future)
   interactions with transport protocols to respond correctly to
   mobility and multihoming events [TCP-RLCI].

                  |-----------------------|
    --------      |   ----------     ----------
    | HIP  |--    ----|  App v6 |    |  App v4 |
    -------- |    |   ----------     ----------
      |      |    |       | HIT           | LSI
      |    ------------   | AF_INET6      | AF_INET
      |    | resolver |   |               |
      |    ------------   |  sockets API  |        user-space
    --|-------------------|-------------------------------
      | sockets and       |               |        kernel
      | PF_KEY API    ---------           |
      |-------------> |TCP/UDP|<-----------
      |               ---------
      |                   |
    ----------        ---------
    | SAD/SPD|<-----> | ESP   |  {HIT_s, HIT_d} <-> SPI
    ----------        ---------  {HIT_s, HIT_d, SPI} <-> {IP_s,IP_d,SPI}
                          |
                      ---------
                      |  IP   |
                      ---------

    Figure 2: Overview of Typical Implementation Changes to Support HIP

   Legacy applications can continue to use the standard AF_INET6 (for
   IPv6) and AF_INET (for IPv4) sockets API.  IPv6 applications bind
   directly to a Host Identity Tag (HIT), which is a part of IPv6
   address space reserved for ORCHIDs.  IPv4 applications bind to a
   Local Scope Identifier (LSI) that has significance only within a
   host; the HIP layer translates from LSIs and HITs to the IP addresses
   that are still used underneath for HIP base exchange.

2.1.1.  ESP Implementation Extensions

   HIP uses a Bound End-to-End Tunnel (BEET) mode of ESP operation,
   which mixes tunnel-mode semantics with transport-mode syntax.  BEET
   is not supported by all operating system distributions at present, so
   kernel modifications might be needed to obtain true kernel support
   using existing IPsec code.  At the time of writing, the BEET mode has
   been adopted to vanilla Linux and FreeBSD kernels.

   The HIPL project has contributed an IPsec BEET patch for the Linux
   kernel.  The kernel-level support could potentially allow all Linux
   implementations of HIP to run in the user-space and use a common
   interface towards the kernel.

   One inconvenience experienced in current Linux IPsec implementation
   (due to the native IPsec implementation, not HIP specifically) is a
   loss of the first data packet that triggers the HIP association
   establishment.  Instead, this packet should be cached and transmitted
   after the association is established.

2.2.  User-Space Implementations

   HIP can be implemented entirely in the user-space, an approach that
   is essential for supporting HIP on hosts for which operating system
   modifications are not possible.  Even on modifiable operating
   systems, there is often a significant deployment advantage in
   deploying HIP only as a user-space implementation.  All three open-
   source implementations provide user-space implementations and binary
   packages (RPMs, DEBs, self-extracting installers) typical of
   application deployment on the target systems.

   When HIP is deployed in the user-space, some technique is necessary
   to identify packets that require HIP processing and divert them to
   the user-space for such processing and to re-inject them into the
   stack for further transport protocol processing.  A commonly used
   technique is to deploy a virtual device in the kernel such as a
   network tap (TAP) device, although operating systems may provide
   other means for diverting packets to user-space.  Routing or packet
   filtering rules must be applied to divert the right packets to these
   devices.

   As an example, the user-space implementation may install a route that
   directs all packets with destination addresses corresponding to HITs
   or LSIs to such a virtual device.  In the user-space daemon, the ESP
   header and possibly the UDP header is applied, an outer IP address
   replaces the HIT, and the packet is re-sent to the kernel.  In the
   reverse direction, a socket associated to ESP or a UDP port number
   may be used to receive ESP-protected packets.  HIP signaling packets
   themselves may be sent and received by a raw socket bound to the HIP
   number or UDP port when UDP encapsulation is used.

2.3.  Issues Common to Both Implementation Approaches

2.3.1.  User-Space Handling of HITs

   Much initial experimentation with HIP has involved using HITs
   directly in IPv6 socket calls, without any resolution infrastructure
   to learn the HIT based on, for example, a domain name, or to resolve
   the IP address.  To experiment with HIP using HITs requires a priori
   HIT exchange, in the absence of a resolution service.  Manual
   exchange of HITs has been a major inconvenience for experimentation.
   It can be overcome via 1) opportunistic HIP mode (RFC 5201, Section

   4.1.6), 2) storing HITs in DNS AAAA entries and looking them up by
   domain name, 3) name resolution service for HITs such as OpenDHT
   [RFC6537], 4) an ad hoc HIT exchange service to populate files on
   each machine, or 5) support for DNS extensions described in RFC 5205.

   Over time, support for these techniques has varied.  The HIPL project
   has experimented with all of them.  OpenHIP lacks support for option
   2, and HIP4BSD lacks support for options 1 and 3.

   Implementing opportunistic HIP mode in a clean way is challenging, as
   HITs need to be known when an application binds or connects to a
   socket.  Approach 2 has been difficult in practice due to resistance
   of sysadmins to include AAAA entries for HITs in the DNS server, and
   is a non-standards-compliant use of the resource record.  Approach 3
   is being progressed with two independent implementations of a HIP-
   OpenDHT interface.  At the moment, the easiest way for enabling
   experimentation appears to be approach 4 when a shell script based on
   Secure SHell (SSH) and Secure Copy (SCP) can connect to a peer
   machine and copy HITs to the local configuration files.  However,
   this approach is not scalable or secure for the long run.  HIPL
   developers have had positive experiences with alternative 5.

2.3.2.  Opportunistic Mode

   In opportunistic mode, the Initiator starts a base exchange without
   knowledge of the Responder's HIT.  The main advantage of the
   opportunistic mode is that it does not require additional lookup
   infrastructure for HIs [RFC5205] [RFC6537].

   The opportunistic mode also has a few disadvantages.  First, the
   Initiator may not identify the Responder uniquely just based on the
   IP address in the presence of private address realms [RFC5770].
   Second, the Initiator has to settle for a "leap of faith"; that is,
   assume there is no man-in-the-middle attack.  However, this can be
   partially mitigated by using certificates at the Responder side
   [RFC6253] or by prompting the user using a graphical interface to
   explicitly accept the connection [paper.usable-security].

   The opportunistic mode requires only minor changes in the state
   machine of the Responder and small changes for the Initiator
   [paper.leap-of-faith].  While the Responder can just select a
   suitable HIT upon receiving the first HIP base exchange packet (known
   as an "I1") without a predefined HIT for the Responder, the Initiator
   should be more careful in processing the first packet from the
   Responder, known as the "R1".  For example, the Initiator should make
   sure that it can disambiguate simultaneously initiated opportunistic
   base exchanges from each other.

   In the context of the HIPL project, the opportunistic mode has been
   successfully applied at the HIP layer for service registration
   [RFC5203].  HIP4BSD implemented opportunistic mode successfully with
   small modifications to the FreeBSD socket layer to support
   opportunistic mode.  However, the Linux implementation was more
   challenging, as described below.

   The HIPL project experimented with opportunistic mode by interposing
   a shim at two different layers.  In the first approach, an API-based
   shim was implemented to capture socket calls from the application.
   This was somewhat complicated to implement and explicitly enabling an
   individual application (or groups of applications) to use the
   opportunistic mode was required.  In the second approach
   [paper.leap-of-faith], the shim was placed between the network and
   transport layers.  Upon successful base exchange, the shim translated
   IP-based packet flows to HIT-based packet flows by re-injecting the
   translated packets back to the networking stack.

   Unless bypassed for DNS, both of the opportunistic mode
   implementation approaches in HIPL subjected the application(s) to
   undergo opportunistic mode procedures also for DNS requests.  Both
   approaches also implemented an optional "fall back" to non-HIP base
   connectivity if the peer did not support HIP.  The detection of peer
   support for HIP was based on timeouts.  To avoid timeouts completely
   and to reduce the delay to a single Round-Trip Time (RTT) for TCP,
   the project also experimented with TCP-specific extensions
   [thesis.bishaj].

   The OpenHIP project experimented with opportunistic mode through the
   use of an opportunistic (-o) option.  For the Responder, this option
   determines whether or not HIP accepts I1s received with a zeroed
   receiver's HIT.  On the Initiator's side, this option allows one to
   configure a name and LSI in the known Host Identities file.  When the
   HIT field is missing, an I1 is sent with a zeroed receiver's HIT.
   The LSI is needed by an IPv4 application to trigger the association.
   Note that, normally, the LSI used is based on the bottom 24 bits of
   the HIT, but in the case of opportunistic mode, the HIT is unknown;
   thus, the LSI may differ from the HIT.

   As a summary of the opportunistic mode experimentation, it is
   possibly best suited for HIP-aware applications.  Either it can be
   used by HIP itself in registration extensions or by native HIP
   applications [RFC6317].  This way, the inherent security trade-offs
   of the opportunistic mode are explicitly visible to the user through
   the HIP-aware application.

2.3.3.  Resolving HITs to Addresses

   When HIP is used in opportunistic mode, the Initiator does not know
   the Responder's HIT, but it does know its IP address.  In most other
   cases, however, the kernel or applications may know the HITs and not
   the IP addresses; in these cases, an IP address resolution step for
   HITs must take place.

   A few techniques have been experimented with.  First, OpenDHT can
   also use HITs as keys for IP address records.  Second, work by
   Ponomarev has shown that the reverse DNS tree may be used for reverse
   lookups of the ORCHID space [HIT2IP].  Third, the need for resolution
   may trigger some type of HIP bootstrap message, similar in some sense
   to an Address Resolution Protocol (ARP) message (to resolve the HIT).
   The bootstrap (BOS) packet used to be present in the early revisions
   of the HIP base specifications, but it was removed from the final
   specifications due to insufficient interest at the time.  The HIPL
   implementation currently sends an I1 to a link broadcast IP address
   if it doesn't know the IP address of the peer.  It has triggered
   warnings in some Windows hosts running antivirus software that
   classified broadcasts with unknown protocol number as intrusion
   attempts.  The utility of this technique is limited to the local
   link.

2.3.4.  IPsec Management API Extensions

   A generic key management API for IP security is known as PF_KEY API
   [RFC2367].  PK_KEY is a socket protocol family that can be used by
   trusted applications to access the IPsec key engine in the operating
   system.  Users of this interface typically need sysadmin privileges.

   HIP-related extensions to the PF_KEY interface define a new protocol
   IPPROTO_HIP.  Their main functionality is replacing the TCP and UDP
   checksum with a HIP-compatible checksum (because the transport
   pseudoheader is based on HITs) in incoming and outgoing packets.
   Recent Linux kernel versions do not require patching for these
   extensions.

2.3.5.  Transport Protocol Issues

   When an application triggers a HIP base exchange through the
   transport protocol, the first data packet can be lost unless the HIP
   and IPsec implementation is able to buffer the packet until the base
   exchange completes and IPsec SAs are set up.  The loss of the data
   packet when it is a TCP SYN packet results in TCP timeout [RFC6298]
   that unnecessarily delays the application.  A loss of a UDP packet
   can cause even longer timeouts in applications.  Therefore, it was
   found to be important for HIP implementations to support the

   buffering of the packet.  On the other hand, if the HIP base exchange
   or UPDATE takes longer than 1 second, which is the case on
   lightweight devices, a spurious timeout can occur at the transport
   layer.  The HIP implementation could prevent this scenario by
   manipulating timeout values at the transport layer or, alternatively,
   dropping the original or retransmitted duplicate packet.

   The multihoming support in [RFC5206] is intended for the purpose of
   failover, when a host starts using an alternative locator when a
   current locator fails.  However, a host could used this multihoming
   support for load balancing across different locators.  Multihoming in
   this manner could potentially cause issues with transport protocol
   congestion control and loss detection mechanisms.  However, no
   experimental results from using HIP multihoming in this capacity have
   been reported.

   The use of paths with different characteristics can also impact the
   estimate of a retransmission timer at the sender's transport layer.
   TCP uses a smoothed average of the path's Round-Trip Time and its
   variation as the estimate for a retransmission timeout.  After the
   retransmission timer expires, the sender retransmits all outstanding
   packets in go-back-N fashion.

   When multihoming is used for simultaneous data transmission from
   several locators, there can easily be scenarios when the
   retransmission timeout does not correspond to the actual value.  When
   packets simply experience different RTT, its variation is high, which
   sets the retransmission timeout value unnecessarily high.  When
   packets are lost, the sender waits excessively long before
   retransmitting.  Fortunately, modern TCP implementations deploying
   Selective Acknowledgments (SACKs) and Limited Transmit are not
   relying on retransmission timeouts except when most outstanding
   packets are lost.

   Load balancing among several paths requires some estimate of each
   path's capacity.  The TCP congestion control algorithm assumes that
   all packets flow along the same path.  To perform load balancing, the
   HIP layer can attempt to estimate parameters such as the delay,
   bandwidth, and loss rate of each path.  A HIP scheduler could then
   distribute packets among the paths according to their capacity and
   delay, to maximize overall utilization and minimize reordering.  The
   design of the scheduler is a topic of current research work; none are
   reported to exist.  Different network paths can have different
   Maximum Transmission Unit (MTU) sizes.  Transport protocols perform
   MTU discovery typically only in the beginning of a connection.  As
   HIP hides mobility from the transport layer, it can happen that
   packets on the new path get fragmented without knowledge of the
   transport protocol.  To solve this problem, the HIP layer could

   inform the transport layer of mobility events.  Protocols to support
   such notifications to the transport layer have been proposed to the
   IETF in the past, including transport triggers [TRIGTRAN],
   lightweight mobility detection and response (LMDR) [LMDR], and TCP
   response to connectivity change [TCP-RLCI].

2.3.6.  Legacy NAT Traversal

   Legacy NAT traversal for outbound-initiated connections to a publicly
   addressed Responder has been implemented by all three HIP
   implementations; two (HIPL and HIP4BSD) implement Interactive
   Connectivity Establishment (ICE) techniques [RFC5770] for inbound NAT
   traversal.  It has also been reported that the use of Teredo
   [RFC4380] over HIP was simpler than the modifications required for
   ICE techniques because Teredo effectively manifests itself as a
   routable, virtual locator to the system.  UDP encapsulation is now
   the default mode of HIP operation for OpenHIP's IPv4 HIP
   implementation.  Finding an IPv6 NAT implementation for experiments
   has been difficult.  In addition, the initial implementations of NAT
   traversal for HIP based on ICE techniques proved to be complicated to
   implement or integrate, and a native NAT traversal mode is now under
   development for HIP [NAT-TRAVERSAL].  NAT traversal is expected to be
   a major mode of HIP operation in the future.

2.3.7.  Local Management of Host Identity Namespace

   One issue not being addressed by some experimental implementations is
   how to perform source HIT selection across possibly multiple host
   identities (some may be unpublished).  This is akin to source address
   selection for transport sockets.  How much HIP policy to expose to
   users is a user interface issue.  Default or automatic configuration
   guesses might have undesirable privacy implications for the user.

   Helsinki University of Technology (TKK, now Aalto) has implemented an
   extension of the native HIP API to control multiple host identities
   [thesis.karlsson].  A problem with Linux routing and multiple
   identities was discovered by the HIPL development group.  As Linux
   routing is based on longest prefix match, having multiple HITs on
   virtual devices is problematic from the viewpoint of access control
   because the stack selects the source HIT based on the destination
   HIT.  A coarse-grained solution for this is to terminate the longest
   prefix match for ORCHIDs in the Linux networking statck.  However, a
   more fine-grained solution tries to return a source HIT matching to
   the algorithm used for generating the destination HIT in order to
   facilitate compatibility with new algorithms standardized in the
   future.

   There are security and privacy issues with storing private keys
   securely on a host.  Current implementations simply store private
   keys in a file that is readable only by applications with root
   privileges.  This may not be a sufficient level of protection, as
   keys could be read directly from the disk or, e.g., some application
   with a set-user-id flag.  Keys may be stored on a trusted platform
   module (TPM), but there are no reported HIP experiments with such a
   configuration.  In a Boeing pilot project, temporary certificates
   were generated from a key on a USB SIM chip and used in the HIP base
   exchange.  Use of certificates in HIP requires extensions to the HIP
   specifications [RFC6253].  Another option is encrypting keys on disks
   and keeping a passkey in memory (like in Secure Socket Layer (SSL)
   certificates on servers, that ask for a password when booting Linux).

2.3.8.  Interactions with Host Firewalls

   HIP is presently an experimental protocol, and some default firewall
   configuration scripts on popular Linux distributions do not permit
   the HIP number.  Determining which rules to modify without
   compromising other policies can be tricky; the default rule set on a
   previous SuSE Linux distribution was discovered to contain over one
   hundred rules.  Moreover, it may be the case that the end user has no
   control over the firewall settings, if administered by an enterprise
   IT department.  However, the use of HIP over UDP has alleviated some
   of these concerns.  When using HIP over UDP, the firewall needs to
   allow outbound UDP packets and responses to them.

2.4.  IPv4 versus IPv6 Issues

   HIP has been oriented towards IPv6 deployment, but all
   implementations have also added support for IPv4.  HIP supports IPv6
   applications well, as the HITs are used from the general IPv6 address
   space using the ORCHID prefix.  HITs are statistically unique,
   although they are not routable at the IP layer.  Therefore, a mapping
   between HITs and routable IP addresses is necessary at the HIP layer,
   unless an overlay network or broadcast technique is available to
   route packets based on HITs.

   For IPv4 applications, a 32-bit Local Scope Identifier (LSI) is
   necessary at the sockets API.  The LSI is an alias for a host
   identity and is only meaningful within one host.  Note that an IPv4
   address may be used as an LSI if it is configured to refer to a
   particular host identity on a given host, or LSIs may be drawn from
   an unallocated IPv4 address range, but lack of coordination on the
   LSI space may hinder implementation portability.

   HIP makes it possible to use IPv6 applications over the IPv4 network
   and vice versa.  This has been called "interfamily operation"
   (flexibility between different address families) and is enabled by
   the fact that the transport pseudoheader is always based on HITs
   regardless of whether the application or the underlying network path
   is based on IPv4.  All three open source HIP implementations have
   demonstrated some form of interfamily handoff support.  The
   interfamily portion of the BEET patch in the Linux kernel was found
   more difficult to complete compared with the single-family
   processing.

   HIP also provides the potential to perform cross-family support,
   whereby one side of a transport session is IPv6 based and another is
   IPv4 based [paper.handovers].

2.5.  What Have Early Adopters Learned from Experience?

   Implementing HIP in current stacks or as overlays on unmodified
   stacks has generally been successful.  Below are some caveats and
   open issues.

   Experimental results comparing a kernel versus user-space HIP
   implementations in terms of performance and DoS resilience would be
   useful.  If the kernel implementation is shown to perform
   significantly better than the user-space implementation, it may be a
   sufficient justification to incorporate HIP within the kernel.
   However, experiences on general purpose laptops and servers suggests
   that for typical client use of HIP, user-space implementations
   perform adequately.

   Although the HIPL kernel-based keying implementation was submitted to
   the Linux kernel development process, the implementation was not
   accepted.  The kernel developers felt that since Mobile IP (MIP) and
   the Internet Key Exchange Protocol (IKE) are implemented as user-
   space signaling daemons in Linux, that should be the approach for
   HIP, too.  Furthermore, the kernel patch was somewhat big, affecting
   the kernel in many places and having several databases.  The Linux
   kernel maintainers did eventually accept the BEET patch.

   Some users have been explicitly asking about the coexistence of HIP
   with other VPN and Mobile IP software.  On Windows, VPN clients tend
   to install their own versions of TAP drivers that might conflict with
   the driver used by the OpenHIP implementation.  There may also be
   issues due to lack of coordination leading to unintended HIP-over-VPN
   sessions or lack of coordination of the ESP Security Parameter Index
   (SPI) space.  However, these types of conflicts are only speculation

   and were not reported to the research group; only some positive
   reports of HIP and VPN software properly coexisting have been
   reported by the HIPL group.

   With legacy applications, LSI support is important because IPv6 is
   not widely used in applications.  The main issues in getting
   applications to work well over HIP have been related to bugs in the
   implementations themselves, or latency related issues (such as TCP
   timeouts due to Linux IPsec implementation).  There have been no
   major obstacles encountered in practice, and there has also been some
   experience in using HIP with native applications [paper.p2psip].

3.  Infrastructure Implications

   This section focuses on the deployment of infrastructure to support
   HIP hosts.

3.1.  Impact on DNS

   HIP DNS extensions [RFC5205] were developed by NEC Eurolabs and
   contributed to OpenHIP and were also developed by the HIPL project,
   both for the BIND9 DNS server.  Legacy applications do not query for
   HIP resource records, but DNS proxies (local resolvers) interpose
   themselves in the resolution path and can query for HI records.  The
   BIND 9 deployment for HIPL uses binary blob format to store the HIP
   resource records; this means that no changes to the DNS server are
   required.

   There have been no studies reported on the impact of changes based on
   [RFC5205] to HIP on the existing DNS.  There have been some studies
   on using DNS to store HITs in the reverse tree [HIT2IP].

3.2.  HIP-Aware Middleboxes

   A design of a HIP registration protocol for architectured NATs (NATs
   that are HIP aware and use HIP identifiers to distinguish between
   hosts) has been completed and published as RFC 5204.  Performance
   measurement results with a prototype are available, but
   experimentation on a wide scale is still missing.  RFC 5207 provides
   a problem statement for HIP-aware NATs and middleboxes [RFC5207].

   As argued by Aura, et al. [paper.hipanalysis], the encryption of the
   Initiator Host Identity (HI) prevents policy-based NAT and firewall
   support, and middlebox authentication, for HIP.  The catch is that
   when the HI is encrypted, middleboxes in the network cannot verify
   the signature of the second base exchange packet from the Initiator

   (I2) and, thus, cannot safely create a state for the HIP association.
   On the other hand, if the HI is not encrypted, a stateful middlebox
   can process the I2 and create protocol state for the session.

3.3.  HIT Resolution Infrastructure

   OpenDHT HIT-to-IP address resolution has been implemented by Aalborg
   University, Denmark, Helsinki Institute for Information Technology
   for HIPL, and by Boeing for OpenHIP [RFC6537].

   The prototype of the Host Identity Indirection Infrastructure (Hi3)
   has been implemented using OpenHIP and HIPL.  A set of 25 i3 servers
   was running on PlanetLab for several years.  While a PlanetLab
   account is required to run the servers, anybody could openly use the
   provided service.

   The main idea of Hi3 is to transmit HIP control packets using the i3
   system as a lookup and rendezvous service, while transmitting data
   packets efficiently end-to-end using IPsec.  Performance measurements
   were conducted comparing the association setup latency, throughput,
   and RTT of Hi3 with plain IP, HIP, and i3 [paper.hi3].

   One difficulty has been with debugging the i3 system.  In some cases,
   the messages did not traverse i3 correctly, due to its distributed
   nature and lack of tracing tools.  Making the system work has been
   challenging.  Further, since the original research work was done, the
   i3 servers have gone offline.

   NATs and firewalls have been a major disturbance in Hi3 experiments.
   Many networks did not allow incoming UDP packets to go through,
   therefore, preventing messages from i3 servers to reach the client.

   So far, the Hi3 system has been evaluated on a larger scale only
   analytically.  The problem is that running a larger number of clients
   to create a sufficient load for the server is difficult.  A cluster
   on the order of a hundred Linux servers is needed for this purpose.
   Contacts to a State Supercomputer Centre in Finland have not been
   successful so far.  A possible option is to use one of the existing
   Emulab installations, e.g., in Utah, for these tests.

3.4.  Rendezvous Servers

   A rendezvous server (RVS) [RFC5204] has been implemented by HIIT for
   HIPL, and an implementation also exists for OpenHIP.  The concept has
   been extended to a relay server in [RFC5770].  Initial
   experimentation with the HIPL implementation produced the following
   observations:

   o  RVS may be better than dynamic DNS updates for hosts that change
      their address rapidly.

   o  Registration of a HIP host to RVS costs a base exchange.

   o  UPDATE and CLOSE packets sent through rendezvous servers is
      advised; RVS handling of UPDATE messages can typically solve the
      double jump [MULTI-HOMED] mobility problem.

   The following advanced concepts need further study:

   o  Multiple RVSs per host for fault tolerance (e.g., one rendezvous
      node crashes) and an algorithm for selecting the best RVS.

   o  Load balancing.  An RVS server could distribute I1s to different
      Responders if the Responder's identity is shared or opportunistic
      HIP is used.

   o  Offering a rendezvous service in a P2P fashion by HIP hosts.

3.5.  Hybrid DNS-DHT Resolution

   In addition to pure DNS and pure DHT HIP name resolution, a hybrid
   approach combining the standard DNS interface for clients with last-
   hop DHT resolution was developed.  The idea is that the benefits of
   DNS solution (wide deployment, support for legacy applications) could
   be combined with advantages of DHT (fault tolerance, efficiency in
   handling flat data keys).  The DHT is typically run internally by the
   organization managing the last-hop DNS zone and the DNS server.  That
   way, the HITs belonging to that organization could be stored locally
   by the organization that improves deployability of the resolution
   system.  However, organizations could also share a DHT between
   themselves or connect their DNS servers to a publicly available DHT,
   such as OpenDHT.  The benefit of running a DHT on a local server
   cluster compared to a geographically spread DHT is higher performance
   due to decreased internal DHT latencies.

   The system was prototyped by modifying the BIND DNS server to
   redirect the queries for HITs to a DHT server.  The interface was
   implemented in XML according to specifications [RFC6537].  The system
   is completely backward compatible to legacy applications since the
   standard DNS resolver interface is used.

   Performance of the system was evaluated by performing a rapid
   sequence of requests for querying and updating the HIT-to-IP address
   mapping.  The request rate was varied from 1 to 200 requests per
   second.  The average latency of one query request was less than 50 ms
   and the secured updated latency less than 100 ms with a low request

   rate.  However, the delay was increasing exponentially with the
   request rate, reaching 1 second for 200 requests per second (update
   rate 0) and almost 2 seconds (update rate 0.5).  Furthermore, the
   maximum delay exceeded the mean by several times.

   Based on experiments, a multi-processor system could handle more than
   1000 queries per second.  The latencies are dominated by the DHT
   resolution delay, and the DNS component is rather small.  This is
   explained by the relative inefficiency of used DHT implementation
   (Bamboo) and could be definitely improved in the future.

4.  Application Implications

   In a deployed HIP environment, applications may be HIP aware or HIP
   unaware.  RFC 5338 [RFC5338] describes various techniques to allow
   HIP to support unmodified applications.  Some additional application
   considerations are listed below.

4.1.  Non-Intrusive HIP Insertion

   One way to support legacy applications that use dynamic linking is to
   dynamically interpose a modified resolver library.  Using HIPL,
   several legacy applications were shown to work without changes using
   dynamic re-linking of the resolver library.  For example, the Firefox
   web browser successfully worked with an Apache web server.  The re-
   linking just requires configuring an LD_PRELOAD system variable that
   can be performed in a user shell profile file or as a start-up
   wrapper for an application.  This provides the user with fine-grained
   policy control over which applications use HIP, which could
   alternately be considered a benefit or a drawback depending on
   whether the user is burdened with such policy choices.  The technique
   was also found to be sensitive to loading LD_PRELOAD twice, in which
   case the order of linking dynamic libraries must be coded carefully.

   Another method for transparently using HIP, which has no reported
   implementation experience, is via local application proxies (e.g.,
   squid web proxy) that are modified to be HIP aware.  Discussion of
   proxies for HIP is a current focus of research group activities
   [HIPRG-PROXIES].

4.2.  Referrals

   A concern that FTP would not work due to the problem of application
   referrals, i.e., passing the IP address within application messages,
   was discovered not to be a problem for FTP in practice.  It is shown
   to work well both in the passive and active modes [paper.namespace].
   It remains an open question how big problem referrals really are in

   the practice.  At least, they do not seem used for the client side
   because they are behind NATs, and, therefore, client addresses are
   unsuitable as referrals.

4.3.  Latency

   Some applications may be sensitive to additional RTTs or processing
   due to HIP resolutions or the protocol itself.  For instance, page
   load speed for web browsers is a critical metric for browser
   designers.  Some applications or deployments may not wish to trade
   application speed for the security and mobility management that HIP
   offers.

5.  Network Operator's Perspective

   There is no known deployment of HIP by a data service provider.
   However, some issues regarding HIP have been brought to the HIP
   research group by a network provider and are summarized below and in
   [HIP-OPERATORS].

5.1.  Management of the Host Identity Namespace

   When a network operator deploys HIP for its customers, several issues
   with management of host identities arise.  The operator may prefer to
   generate the host identity itself rather than let each host create
   the identities.  Several factors can create such a need.  Public-
   private key generation is a demanding operation that can take tens of
   seconds on a lightweight device, such as a mobile phone.  After
   generating a host identity, the operator can immediately insert it
   into its own AAA databases and network firewalls.  This way, the
   users would not need to be concerned with technical details of host
   identity management.

   The operator may use a Public Key Infrastructure (PKI) to certify
   host identities of its customers.  Then, it uses the private key of
   an operator's Certificate Authority (CA) to sign the public key of
   its customers.  This way, third parties possessing the public key of
   the CA can verify the customer's host identity and use this
   information, e.g., for admission control to infrastructure.  Such
   practice raises the security level of HIP when self-generated host
   identities are used.

   When the operator is using neither PKI nor DNS Security (DNSSEC) host
   names, the problem of securely exchanging host identities remains.
   When HIP is used in opportunistic mode, a man-in-the-middle can still
   intercept the exchange and replace the host identities with its own.

   For instance, the signaling provided by SIP could be used to deliver
   host identities if it were secured by existing mechanisms in the
   operator's network.

5.2.  Use of ESP Encryption

   The research group has discussed whether operators can provide
   "value-added" services and priority, and comply with wiretapping
   laws, if all sessions are encrypted.  This is not so much a HIP issue
   as a general end-to-end encryption issue.

   The processing power of mobile devices also must be considered.  One
   study evaluated the use of HIP and ESP on lightweight devices (Nokia
   N770 Internet Tablets having 200 MHz processors) [paper.mobiarch].
   The overhead of using ESP on such a platform was found to be
   tolerable, about 30% in terms of throughput.  With a bulk TCP
   transfer over WiFi, transfer without HIP was producing 4.86 Mbps,
   while over ESP security associations set up by HIP it was 3.27 Mbps.
   A lightweight HIP base exchange for this purpose is being developed
   at the time of this writing [HIP-DEX].

   It is also possible to use HIP in a NULL encryption configuration if
   one of SHA1 or MD5 authentication are used.

5.3.  Access Control Lists Based on HITs

   A firewall typically separates an organization's network from the
   rest of the Internet.  An Access Control List (ACL) specifies packet
   forwarding policies in the firewall.  Current firewalls can filter
   out packets based on IP addresses, transport protocol, and port
   values.  These values are often unprotected in data packets and can
   be spoofed by an attacker.  By trying out common well-known ports and
   a range of IP addresses, an attacker can often penetrate the firewall
   defenses.

   Furthermore, legacy firewalls often disallow IPsec traffic and drop
   HIP control packets.  HIP allows ACLs to be protected based on packet
   exchanges that may be authenticated by middleboxes.  However, HITs
   are not aggregatable, so HIT-based ACLs may be longer in length (due
   to an inability to group hosts with a single entry) and harder to
   deal with by human users (due to the length of the HIT compared with
   an IPv4 or IPv6 prefix).

   Additionally, operators would like to grant access to the clients
   from domains such as example.com regardless of their current locators
   or HITs.  This is difficult without a forward confirmed reverse DNS
   to use for non-repudiation purposes.

5.4.  Firewall Issues

   Helsinki University of Technology (TKK, now Aalto) has implemented a
   HIP firewall based on Linux iptables [paper.firewall] that operates
   in user-space.

   In general, firewalls can be stateless, filtering packets based only
   on the ACL, and stateful, following and remembering packet flows.
   Stateless firewalls are simple to implement but provide only coarse-
   grained protection.  However, their performance can be efficient
   since packet processing requires little memory or CPU resources.  A
   stateful firewall determines if a packet belongs to an existing flow
   or starts a new flow.  A flow identifier combines information from
   several protocol headers to classify packets.  A firewall removes the
   state when the flow terminates (e.g., a TCP connection is closed) or
   after a timeout.  A firewall can drop suspicious packets that fail a
   checksum or contain sequence numbers outside of the current sliding
   window.

   A transparent firewall does not require that hosts within the
   protected network register or even know of the existence of the
   firewall.  An explicit firewall requires registration and
   authentication of the hosts.

   A HIP-aware firewall operating in the middle identifies flows using
   HITs of communicating hosts, as well as SPI values and IP addresses.
   The firewall must link together the HIP base exchange and subsequent
   IPsec ESP data packets.  During the base exchange, the firewall
   learns the SPI values from I2 and R2 packets.  Then, the firewall
   only allows ESP packets with a known SPI value and arriving from the
   same IP address as during the base exchange.  If the host changes its
   location and the IP address, the firewall, if still on the path,
   learns about the changes by following the mobility update packets.

   It is possible to implement a stateless, end-host-based firewall to
   reuse existing higher-layer mechanisms such as access control lists
   in the system.  In this mode of operation, HITs would be used in the
   access control lists, and while the base exchange might complete, ESP
   is not passed to the transport layer unless the HITs are allowed in
   the access control list.

   A HIP host can register to an explicit firewall using the usual
   procedure [RFC5203].  The registration enables the host and the
   firewall to authenticate each other.  In a common case, where the
   Initiator and Responder hosts are located behind different firewalls,
   the Initiator may need to first register with its own firewall, and
   afterward, with the Responder's firewall.

   Some researchers have suggested that a firewall for security-critical
   environments should get involved in the base exchange and UPDATE
   procedures with middlebox-injected echo requests.  Otherwise, the
   firewall can be circumvented with replay attacks if there is a
   compromised node within the network that the firewall is trying to
   protect [HIP-MIDDLE].

6.  User Privacy Issues

   Using public keys for identifying hosts creates a privacy problem as
   third parties can determine the source host even if attached to a
   different location in the network.  Various transactions of the host
   could be linked together if the host uses the same public key.
   Furthermore, using a static IP address also allows linking of
   transactions of the host.  Multiplexing multiple hosts behind a
   single NAT or using short address leases from DHCP can reduce the
   problem of user tracking.  However, IPv6 addresses could reduce the
   occurrence of NAT translation and cause additional privacy issues
   related to the use of Media Access Control (MAC) addresses in IPv6
   address autoconfiguration.  HIP does provide for the use of anonymous
   (unpublished) HITs in cases in which the Initiator prefers to remain
   anonymous, but the Responder must be willing to accept sessions from
   anonymous peers.

   With mutual authentication, the HIP Initiator should not have to
   reveal its identity (public key) to either a passive adversary or an
   active attacker.  The HIP Initiator can authenticate the Responder's
   R1 packet before encrypting its host identity with the Diffie-
   Hellman-generated keying material and sending it in the I2 packet.
   The authentication step upon receiving an R1 defeats the active
   attacker (impersonator) of the Responder, and the act of encrypting
   the identity defeats the passive adversary.  Since the Responder
   sends its public key unencrypted in the first reply message (R1) to
   the Initiator, the Responder's identity will be revealed to third-
   party on-path eavesdroppers.  However, if the Responder authenticates
   the Initiator and performs access controls before sending the R1, the
   Responder can avoid disclosing its public key to an active attacker.

   DNS records can provide information combining host identity and
   location information, the host public key, and host IP address.
   Therefore, identity and location privacy are related and should be
   treated in an integrated approach.  The goal of the BLIND is to
   provide a framework for identity and location privacy [paper.blind]
   [HIP-PRIVACY].  The identity protection is achieved by hiding the
   actual public keys from third parties so that only the trusted hosts
   can recognize the keys.  Location privacy is achieved by integrating
   traffic forwarding with NAT translation and decoupling host
   identities from locators.  The use of random IP and MAC addresses

   also reduces the issue of location privacy shifting the focus to
   protecting host identifiers from third parties.  This approach is, by
   its very nature, incompatible with middlebox authentication.

   To prevent revealing the identity, the host public key and its hash
   (HIT) can be encrypted with a secret key known beforehand to both
   Initiator and Responder.  However, this is a requirement that cannot
   be easily implemented in practice.  The BLIND framework provides
   protection from active and passive attackers using a modified HIP
   base exchange.  If the host avoids storing its public keys in the
   reverse DNS or DHT repository, the framework achieves full location
   and identity privacy.

   An alternative approach to reducing privacy threats of persistent
   identifiers is to replace them with short-lived identifiers that are
   changed regularly to prevent user tracking.  Furthermore, identifiers
   must be changed simultaneously at all protocol layers; otherwise, an
   adversary could still link the new identifier by looking at an
   identifier at another protocol layer that remained the same after the
   change.  The HIP privacy architecture that simultaneously changes
   identifiers on MAC, IP, and HIP/IPsec layers was developed at
   Helsinki University of Technology (TKK, now Aalto) [thesis.takkinen].
   HIP could be extended in the future to allow active sessions to
   migrate identities.

7.  Experimental Basis of This Report

   This report is derived from reported experiences and research results
   of early adopters, implementers, and research activities.  In
   particular, a number of implementations have been in development
   since 2002 (Section 2).

   One production-level deployment of HIP has been reported.  Boeing has
   described how it uses HIP to build Layer 2 VPNs over untrusted
   wireless networks [HIPLS].  This use case is not a traditional end-
   host-based use of HIP, but rather, it is one that uses HIP-aware
   middleboxes to create ESP tunnels on-demand between provider-edge
   (PE) devices.

   The InfraHIP II project is deploying HIP infrastructure (test
   servers, rendezvous and relay servers) in the public Internet.

   The following is a possibly incomplete list of past and current
   research activities related to HIP.

   o  Boeing Research & Technology (J. Ahrenholz, O. Brewer, J. Fang, T.
      Henderson, D. Mattes, J. Meegan, R. Paine, S. Venema, OpenHIP
      implementation, Secure Mobile Architecture)

   o  NomadicLab, Ericsson (P. Jokela, P. Nikander, J. Melen.  BSD HIP
      implementation)

   o  Helsinki Institute for Information Technology (HIIT) (A. Gurtov,
      M. Komu, A. Pathak, D. Beltrami.  HIPL, legacy NAT traversal,
      firewall, i3, native API)

   o  Helsinki University of Technology (TKK, now Aalto) (Janne
      Lindqvist, Niklas Karlsson, Laura Takkinen, and Essi Vehmersalo.
      HIP security and firewalls, multiple identities, and privacy
      management)

   o  University of California, Berkeley (A. Joseph, HIP proxy
      implementation)

   o  Laboratory of Computer Architecture and Networks, Polytechnic
      School of University of Sao Paulo, Brazil (T. Carvalho, HIP
      measurements, Hi3)

   o  Telecom Italia (M. Morelli, comparing existing HIP
      implementations)

   o  NEC Heidelberg (L. Eggert, M. Esteban, V. Schmitt working on RVS
      implementation, DNS, NAT traversal)

   o  University of Hamburg-Harburg (M. Shanmugam, A. Nagarajan, HIP
      registration protocol)

   o  University of Tuebingen (K. Wehrle, T. Lebenslauf to work on Hi3
      or HIP-OpenDHT)

   o  University of Parma (UNIPR), Department of Information Engineering
      Parma, Italy.  (N. Fedotova, HIP for P2P)

   o  Siemens (H. Tschofenig, HIP middleboxes)

   o  Denmark (Aalborg University, Lars Roost, Gustav Haraldsson, Per
      Toft, HIP evaluation project, OpenDHT-HIP interface)

   o  Microsoft Research, Cambridge (T. Aura, HIP analysis)

   o  MIT (H. Balakrishnan.  Delegation-Oriented Architecture)

   o  Huawei (D. Zhang, X. Xu, hierarchical HIP architecture, HIP proxy,
      key revocation)

8.  Related Work on ID-Locator Split

   This section briefly summarizes the related work on the ID-locator
   split with particular focus on recent IETF and IRTF activity.  In the
   academic research community, several related proposals were explored
   prior to the founding of this research group, such as the Internet
   Indirection Infrastructure (i3) [paper.i3], IPNL [paper.layered],
   DataRouter [paper.datarouter], Network Pointers [paper.netpointers],
   FARA [paper.fara], and TRIAD [paper.triad].

   The topic of whether a new namespace is needed for the Internet has
   been controversial.  The Namespace Research Group (NSRG) at the IRTF
   was not able to reach consensus on the issue, nor even to publish a
   final report.  Yet, there seems to be little disagreement that, for
   many scenarios, some level of indirection from network name to
   network location is essential or highly desirable to provide adequate
   service.  Mobile IP [RFC6275] is one example that reuses an existing
   namespace for host naming.  Since Mobile IP was finalized, many new
   variants to providing this indirection have been suggested.  Even
   prior to Mobile IP, the IETF has published informational documents
   describing architectures separating network name and location,
   including the work of Jerome Saltzer [RFC1498] and Nimrod [RFC1992].

   Most recently, there have been standardization and development
   efforts in the IETF and IRTF as follows:

   o  The Site Multihoming in IPv6 (multi6) WG documented the ways that
      multihoming is currently implemented in IPv4 networks and
      evaluated several approaches for advanced multihoming.  The
      security threats and impact on transport protocols were covered
      during the evaluation.  The work continued in another WG, Site
      Multihoming by IPv6 Intermediation (shim6), which is focusing on
      specifications of one selected approach [RFC5533].  Shim6 uses the
      approach of inserting a shim layer between the IP and the
      transport layers that hides effects of changes in the set of
      available addresses.  The applications are using one active
      address that supports referrals.  Shim6 relies on
      cryptographically generated IPv6 addresses to solve the address
      ownership problem.  HIP and shim6 are architecturally similar and
      use a common format for control packets.  HIP specifications
      define only simple multihoming scenarios leaving such important
      issues as interface selection untouched.  Shim6 offers
      complementary functionality that can be reused in HIP [REAP4HIP].
      The OpenHIP implementation integrates HIP and shim6 protocols in
      the same framework, with the goal of allowing HIP to reuse the
      shim6 failure detection protocol.  Furthermore, HIP and shim6
      socket APIs have been jointly designed [RFC6317] [RFC6316].

   o  The IRTF Routing Research Group (RRG) has explored a class of
      solutions to the global routing scalability problem that involve
      either separation of the existing IP address space into those used
      for identifiers and locators as in LISP [LISP] and Six/One Router
      [SIX-ONE] and those advocating a fuller separation of these roles
      including ILNP [ILNP] and RANGI [RANGI].

   o  The End-Middle-End research group considered the potential for an
      explicit signaling and policy control plane for middleboxes and
      endpoints [EME]; at a joint meeting at IETF 69, the HIP and EME
      research groups discussed whether the EME framework could help HIP
      with middlebox traversal.

   o  The IETF Multipath TCP working group is developing mechanisms to
      simultaneously use multiple paths in a regular TCP session.  The
      MPTCP solution aims to solve the multihoming problem also
      addressed by HIP but by solving it for TCP specifically.

   o  The Unmanaged Internet Protocol bears several similarities to the
      HIP architecture, such as the focus on identifiers that are not
      centrally managed that are also based on a cryptographic hash of a
      node's public key [thesis.ford].

   o  Apple Back To My Mac service provides secure connections between
      hosts using IPsec between a pair of host identifiers.  However,
      the host identifier is reported to be an IPv6 Unique Local
      Addressing (ULA) address rather than a HIP identifier [RFC6281].

   Although the HIP research group has not formally tried to compare HIP
   with other ID-locator split approaches, such discussions have
   occurred on other lists such as the Routing research group mailing
   list, and a comparison of HIP's mobility management solution with
   other approaches was published in [MOBILITY-COMPARISON].

9.  Security Considerations

   This document is an informational survey of HIP-related research and
   experience.  Space precludes a full accounting of all security issues
   associated with the approaches surveyed here, but the individually
   referenced documents may discuss security considerations for their
   respective protocol component.  HIP security considerations for the
   base HIP protocol can be found in Section 8 of [RFC5201].

10.  Acknowledgments

   Miika Komu, Pekka Nikander, Ari Keranen, and Jeff Ahrenholz have
   provided helpful comments on earlier draft versions of this document.
   Miika Komu also contributed the section on opportunistic mode.  We
   also thank Dacheng Zhang for contributions on hierarchical HIP
   architectures and the Crypto Forum Research Group (Adam Back and Paul
   Hoffman) for clarification of Diffie-Hellman privacy properties.

11.  Informative References

   [BEET-MODE] Nikander, P. and J. Melen, "A Bound End-to-End Tunnel
               (BEET) mode for ESP", Work in Progress, August 2008.

   [EME]       Francis, P., Guha, S., Brim, S., and M. Shore, "An EME
               Signaling Protocol Design", Work in Progress, April 2007.

   [HIP-DEX]   Moskowitz, R., "HIP Diet EXchange (DEX)", Work
               in Progress, March 2011.

   [HIP-MIDDLE]
               Hummen, R., Heer, T., Wehrle, K., and M. Komu, "End-Host
               Authentication for HIP Middleboxes", Work in Progress,
               October 2011.

   [HIP-OPERATORS]
               Dietz, T., Brunner, M., Papadoglou, N., Raptis, V., and
               K.  Kypris, "Issues of HIP in an Operators Networks",
               Work in Progress, October 2005.

   [HIP-PRIVACY]
               Zhang, D. and M. Komu, "An Extension of HIP Base Exchange
               to Support Identity Privacy", Work in Progress,
               July 2011.

   [HIPLS]     Henderson, T., Venema, S., and D. Mattes, "HIP-based
               Virtual Private LAN Service (HIPLS)", Work in Progress,
               September 2011.

   [HIPRG-PROXIES]
               Zhang, D., Xu, X., Yao, J., and Z. Cao, "Investigation in
               HIP Proxies", Work in Progress, October 2011.

   [HIT2IP]    Ponomarev, O. and A. Gurtov, "Embedding Host Identity
               Tags Data in DNS", Work in Progress, July 2009.

   [ILNP]      Atkinson, R., "ILNP Concept of Operations", Work
               in Progress, July 2011.

   [LISP]      Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
               "Locator/ID Separation Protocol (LISP)", Work
               in Progress, November 2011.

   [LMDR]      Swami, Y., Le, K., and W. Eddy, "Lightweight Mobility
               Detection and Response (LMDR) Algorithm for TCP", Work
               in Progress, February 2006.

   [MOBILITY-COMPARISON]
               Thaler, D., "A Comparison of IP Mobility-Related
               Protocols", Work in Progress, October 2006.

   [MULTI-HOMED]
               Huitema, C., "Multi-homed TCP", Work in Progress,
               May 1995.

   [NAT-TRAVERSAL]
               Keranen, A. and J. Melen, "Native NAT Traversal Mode for
               the Host Identity Protocol", Work in Progress,
               January 2011.

   [RANGI]     Xu, X., "Routing Architecture for the Next Generation
               Internet (RANGI)", Work in Progress, August 2010.

   [REAP4HIP]  Oliva, A. and M. Bagnulo, "Fault tolerance configurations
               for HIP multihoming", Work in Progress, July 2007.

   [RFC1498]   Saltzer, J., "On the Naming and Binding of Network
               Destinations", RFC 1498, August 1993.

   [RFC1992]  Castineyra, I., Chiappa, N., and M. Steenstrup, "The
               Nimrod Routing Architecture", RFC 1992, August 1996.

   [RFC2367]   McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
               Management API, Version 2", RFC 2367, July 1998.

   [RFC4303]   Kent, S., "IP Encapsulating Security Payload (ESP)",
               RFC 4303, December 2005.

   [RFC4380]   Huitema, C., "Teredo: Tunneling IPv6 over UDP through
               Network Address Translations (NATs)", RFC 4380,
               February 2006.

   [RFC4423]   Moskowitz, R. and P. Nikander, "Host Identity Protocol
               (HIP) Architecture", RFC 4423, May 2006.

   [RFC4843]   Nikander, P., Laganier, J., and F. Dupont, "An IPv6
               Prefix for Overlay Routable Cryptographic Hash
               Identifiers (ORCHID)", RFC 4843, April 2007.

   [RFC5201]   Moskowitz, R., Nikander, P., Jokela, P., and T.
               Henderson, "Host Identity Protocol", RFC 5201,
               April 2008.

   [RFC5202]   Jokela, P., Moskowitz, R., and P. Nikander, "Using the
               Encapsulating Security Payload (ESP) Transport Format
               with the Host Identity Protocol (HIP)", RFC 5202,
               April 2008.

   [RFC5203]   Laganier, J., Koponen, T., and L. Eggert, "Host Identity
               Protocol (HIP) Registration Extension", RFC 5203,
               April 2008.

   [RFC5204]   Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
               Rendezvous Extension", RFC 5204, April 2008.

   [RFC5205]   Nikander, P. and J. Laganier, "Host Identity Protocol
               (HIP) Domain Name System (DNS) Extensions", RFC 5205,
               April 2008.

   [RFC5206]   Nikander, P., Henderson, T., Vogt, C., and J. Arkko,
               "End- Host Mobility and Multihoming with the Host
               Identity Protocol", RFC 5206, April 2008.

   [RFC5207]   Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
               Firewall Traversal Issues of Host Identity Protocol (HIP)
               Communication", RFC 5207, April 2008.

   [RFC5338]   Henderson, T., Nikander, P., and M. Komu, "Using the Host
               Identity Protocol with Legacy Applications", RFC 5338,
               September 2008.

   [RFC5533]   Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
               Shim Protocol for IPv6", RFC 5533, June 2009.

   [RFC5770]   Komu, M., Henderson, T., Tschofenig, H., Melen, J., and
               A.  Keranen, "Basic Host Identity Protocol (HIP)
               Extensions for Traversal of Network Address Translators",
               RFC 5770, April 2010.

   [RFC6253]   Heer, T. and S. Varjonen, "Host Identity Protocol
               Certificates", RFC 6253, May 2011.

   [RFC6275]   Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
               Support in IPv6", RFC 6275, July 2011.

   [RFC6281]   Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
               "Understanding Apple's Back to My Mac (BTMM) Service",
               RFC 6281, June 2011.

   [RFC6298]   Paxson, V., Allman, M., Chu, J., and M. Sargent,
               "Computing TCP's Retransmission Timer", RFC 6298,
               June 2011.

   [RFC6316]   Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto,
               "Sockets Application Program Interface (API) for
               Multihoming Shim", RFC 6316, July 2011.

   [RFC6317]   Komu, M. and T. Henderson, "Basic Socket Interface
               Extensions for the Host Identity Protocol (HIP)",
               RFC 6317, July 2011.

   [RFC6537]   Ahrenholz, J., "Host Identity Protocol Distributed Hash
               Table Interface", RFC 6537, February 2012.

   [SIX-ONE]   Vogt, C., "Six/One: A Solution for Routing and Addressing
               in IPv6", Work in Progress, October 2009.

   [TCP-RLCI]  Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami,
               Y., and K. Le, "TCP Response to Lower-Layer Connectivity-
               Change Indications", Work in Progress, February 2008.

   [TRIGTRAN]  Dawkins, S., Williams, C., and A. Yegin, "Framework and
               Requirements for TRIGTRAN", Work in Progress,
               February 2003.

   [book.gurtov]
               Gurtov, A., "Host Identity Protocol (HIP): Towards the
               Secure Mobile Internet", ISBN 978-0-470-99790-1, Wiley
               and Sons, (Hardcover, p 332), June 2008.

   [paper.blind]
               Ylitalo, J. and P. Nikander, "BLIND: A complete identity
               protection framework for end-points", Proc. of
               the Twelfth International Workshop on Security Protoc
               ols, April 2004.

   [paper.datarouter]
               Touch, J. and V. Pingali, "DataRouter:  A Network-Layer
               Service for Application-Layer Forwarding",  Proceedings
               of International Workshop on Active Networks (IWAN),
               May 2003.

   [paper.fara]
               Clark, D., Braden, R., Falk, A., and V. Pingali, "FARA:
               Reorganizing the Addressing Architecture",  Proceedings
               of ACM SIGCOMM FDNA Workshop, August 2003.

   [paper.firewall]
               Lindqvist, J., Vehmersalo, E., Komu, M., and J. Manner,
               "Enterprise Network Packet Filtering for Mobile
               Cryptographic Identities", International Journal of
               Handheld Computing Research (IJHCR), Volume 1, Issue
               1, Pages 79-94, January 2010.

   [paper.handovers]
               Varjonen, S., Komu, M., and A. Gurtov, "Secure and
               Efficient IPv4/IPv6 Handovers Using Host-Based
               Identifier-Locator Split",  Proceedings of the 17th
               International Conference  Software, Telecommunications,
               and Computer Networks, September 2009.

   [paper.hi3] Gurtov, A., Korzon, D., Lukyanenko, A., and P. Nikander,
               "Hi3: An Efficient and Secure Networking Architecture for
               Mobile Hosts", Computer communication, 31 (2008), p.
               2457- 2467, <http://www.cs.helsinki.fi/u/gurtov/papers/
               comcom_hi3.pdf>.

   [paper.hipanalysis]
               Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of the
               HIP Base Exchange Protocol",  Proc. of the 10th
               Australasian Conference on Information Security and
               Privacy (ACISP), July 2005.

   [paper.i3]  Stoica, I., Adkins, D., Zhuang, S., Shenker, S., and S.
               Surana, "Internet Indirection Infrastructure (i3)",
                Proceedings of ACM SIGCOMM, August 2002.

   [paper.layered]
               Balakrishnan, H., Lakshminarayanan, K., Ratnasamy, S.,
               Shenker, S., Stoica, I., and M. Walfish, "A Layered
               Naming Architecture for the Internet",  Proceedings of
               ACM SIGCOMM, August 2004.

   [paper.leap-of-faith]
               Komu, M. and J. Lindqvist, "Leap-of-faith security is
               enough for IP mobility", Proceedings of the 6th IEEE
               Conference on Consumer Communications and Networking
               Conference (CCNC 09), 2009.

   [paper.mobiarch]
               Khurri, A., Vorobyeva, E., and A. Gurtov, "Performance of
               Host Identity Protocol on Lightweight Hardware",
                Proceedings of ACM MobiArch, August 2007.

   [paper.namespace]
               Komu, M., Tarkoma, S., Kangasharju, J., and A. Gurtov,
               "Applying a Cryptographic Namespace to Applications",
                Proc. of First International ACM Workshop on Dynamic
               Interconnection of Networks, September 2005.

   [paper.netpointers]
               Tschudin, C. and R. Gold, "Network pointers", ACM SIGCOMM
               Computer Communications Review, Vol. 33, Issue 1,
               January 2003.

   [paper.p2psip]
               Koskela, J., Heikkila, J., and A. Gurtov, "A secure P2P
               SIP system with SPAM prevention",  ACM Mobile Computer
               Communications Review, July 2009.

   [paper.triad]
               Cheriton, D. and M. Gritter, "TRIAD: A New
               Next-Generation Internet Architecture", July 2000,
               <http://www-dsg.stanford.edu/triad/triad.ps.gz>.

   [paper.usable-security]
               Karvone, K., Komu, M., and A. Gurtov, "Usable Security
               Management with Host Identity Protocol",  Proc. of the
               IEEE/ACS International Conference on Computer Systems and
               Applications, May 2009.

   [thesis.bishaj]
               Bishaj, B., "Efficient Leap of Faith Security with Host
               Identity Protocol",  Master thesis, Helsinki University
               of Technology, June 2008.

   [thesis.ford]
               Ford, B., "UIA:  A Global Connectivity Architecture for
               Mobile Personal Devices",  Doctoral thesis, Massachusetts
               Institute of Technology, September 2008.

   [thesis.karlsson]
               Karlsson, N., "Enabling Multiple Host Identities on
               Linux",  Master thesis, Helsinki University of
               Technology, September 2005.

   [thesis.takkinen]
               Takkinen, L., "Host Identity Protocol Privacy
               Management", Master thesis, March 2006,
               <http://www.tml.tkk.fi/~anttiyj/Laura-Privacy.pdf>.

Authors' Addresses

   Thomas Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   EMail: thomas.r.henderson@boeing.com

   Andrei Gurtov
   University of Oulu
   Centre for Wireless Communications CWC
   P.O. Box 4500
   FI-90014 University of Oulu
   Finland

   EMail: gurtov@ee.oulu.fi

 

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